package raykernel.util;

import java.math.BigInteger;
import java.util.LinkedList;
import java.util.List;

public class PermutationGenerator<T>
{

	T[] list;

	private final int n;
	private final int[] a;
	private BigInteger numLeft;
	private final BigInteger total;

	//-----------------------------------------------------------
	// Constructor. WARNING: Don't make n too large.
	// Recall that the number of permutations is n!
	// which can be very large, even when n is as small as 20 --
	// 20! = 2,432,902,008,176,640,000 and
	// 21! is too big to fit into a Java long, which is
	// why we use BigInteger instead.
	//----------------------------------------------------------

	public PermutationGenerator(T[] list)
	{
		this.list = list;
		n = list.length;

		if (n < 1)
			throw new IllegalArgumentException("Min 1");
		a = new int[n];
		total = getFactorial(n);
		reset();
	}

	//------
	// Reset
	//------

	public void reset()
	{
		for (int i = 0; i < a.length; i++)
		{
			a[i] = i;
		}
		numLeft = new BigInteger(total.toString());
	}

	//------------------------------------------------
	// Return number of permutations not yet generated
	//------------------------------------------------

	public BigInteger getNumLeft()
	{
		return numLeft;
	}

	//------------------------------------
	// Return total number of permutations
	//------------------------------------

	public BigInteger getTotal()
	{
		return total;
	}

	//-----------------------------
	// Are there more permutations?
	//-----------------------------

	public boolean hasMore()
	{
		return numLeft.compareTo(BigInteger.ZERO) == 1;
	}

	//------------------
	// Compute factorial
	//------------------

	private static BigInteger getFactorial(int n)
	{
		BigInteger fact = BigInteger.ONE;
		for (int i = n; i > 1; i--)
		{
			fact = fact.multiply(new BigInteger(Integer.toString(i)));
		}
		return fact;
	}

	//--------------------------------------------------------
	// Generate next permutation (algorithm from Rosen p. 284)
	//--------------------------------------------------------

	public List<T> getNext()
	{
		int[] calc = calc();

		List<T> out = new LinkedList<T>();

		for (int i = 0; i < n; i++)
		{
			out.add(list[calc[i]]);
		}

		return out;

	}
	public int[] calc()
	{

		if (numLeft.equals(total))
		{
			numLeft = numLeft.subtract(BigInteger.ONE);
			return a;
		}

		int temp;

		// Find largest index j with a[j] < a[j+1]

		int j = a.length - 2;
		while (a[j] > a[j + 1])
		{
			j--;
		}

		// Find index k such that a[k] is smallest integer
		// greater than a[j] to the right of a[j]

		int k = a.length - 1;
		while (a[j] > a[k])
		{
			k--;
		}

		// Interchange a[j] and a[k]

		temp = a[k];
		a[k] = a[j];
		a[j] = temp;

		// Put tail end of permutation after jth position in increasing order

		int r = a.length - 1;
		int s = j + 1;

		while (r > s)
		{
			temp = a[s];
			a[s] = a[r];
			a[r] = temp;
			r--;
			s++;
		}

		numLeft = numLeft.subtract(BigInteger.ONE);
		return a;

	}

	public static void main(String[] args)
	{
		String[] stuff = {"a", "bee", "sea", "dee", "eff"};

		PermutationGenerator<String> pg = new PermutationGenerator<String>(stuff);

		while (pg.hasMore())
		{
			System.out.println(pg.getNext());
		}
	}

}
